Re: [PATCH 3/8] mm, hugetlb: fix race in region tracking

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Sun, Jan 26, 2014 at 07:52:21PM -0800, Davidlohr Bueso wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> There is a race condition if we map a same file on different processes.
> Region tracking is protected by mmap_sem and hugetlb_instantiation_mutex.
> When we do mmap, we don't grab a hugetlb_instantiation_mutex, but only the,
> mmap_sem (exclusively). This doesn't prevent other tasks from modifying the
> region structure, so it can be modified by two processes concurrently.
> 
> To solve this, introduce a spinlock to resv_map and make region manipulation
> function grab it before they do actual work.
> 
> Acked-by: David Gibson <david@gibson.dropbear.id.au>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> [Updated changelog]
> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> ---
...
> @@ -203,15 +200,23 @@ static long region_chg(struct resv_map *resv, long f, long t)
>  	 * Subtle, allocate a new region at the position but make it zero
>  	 * size such that we can guarantee to record the reservation. */
>  	if (&rg->link == head || t < rg->from) {
> -		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> -		if (!nrg)
> -			return -ENOMEM;
> +		if (!nrg) {
> +			spin_unlock(&resv->lock);

I think that doing kmalloc() inside the lock is simpler.
Why do you unlock and retry here?

Thanks,
Naoya Horiguchi

> +			nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> +			if (!nrg)
> +				return -ENOMEM;
> +
> +			goto retry;
> +		}
> +
>  		nrg->from = f;
>  		nrg->to   = f;
>  		INIT_LIST_HEAD(&nrg->link);
>  		list_add(&nrg->link, rg->link.prev);
> +		nrg = NULL;
>  
> -		return t - f;
> +		chg = t - f;
> +		goto out_locked;
>  	}
>  
>  	/* Round our left edge to the current segment if it encloses us. */

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